1. Field of the Invention
The present invention relates generally to the field of magnetic resonance imaging (MRI) coils. More particularly, the present invention relates to the use of additional coils in MRI system design operable for producing fields which counteract fields generated by eddy currents in nearby conducting structures.
2. Description of the Related Art
Magnetic resonance imaging (MRI) is a widely accepted and commercially available technique for obtaining digitized visual images representing the internal structures of objects, such as the tissues of the human body, having substantial populations of atomic nuclei that are susceptible to nuclear magnetic resonance (NMR) phenomena. In MRI, the nuclei in a structure to be imaged are polarized by imposing a strong, uniform magnetic field on the nuclei. Selected nuclei are then excited by imposing a radio frequency (RF) signal at a predetermined NMR frequency. By doing this repeatedly while applying different magnetic field gradients and suitably analyzing the resulting RF responses from the nuclei, a map or image of the relative NMR responses as a function of nuclei location may be determined. Data representing the NMR responses in space may be displayed.
The static magnetic field must be very stable and very strong. Typically, the z-axis is parallel to the axis of the main magnetic field for systems in which the magnet has cylindrical geometry, such as for whole subject imaging. The gradient coils create a magnetic field within the coil with a linear spatial gradient, also referred to as a magnetic gradient. Two different types of gradient coils are typically used to produce magnetic gradients for MRI, one for creating a magnetic gradient along the longitudinal axis of the coil (z axis), and a second for creating magnetic gradients along either the x or y transverse axes.
A conventional MRI system further includes a metallic cylinder referred to as a bore tube. The volume inside of the bore tube is referred to as the image volume. Most imaging occurs in the central portion of the bore tube. The current in the gradient coils induces eddy currents in the bore tube and other conducting structures of the magnet of the MRI system that, in turn, induce a magnetic field within the image volume, referred to as the error field. The magnetic field created by the eddy currents is undesirable in the image volume. The gradient fields must be well defined during the encoding process or the image produced will be distorted. In order to obtain a clear image, it is necessary to reduce or eliminate these eddy current fields.
Conventional attempts at reducing eddy currents include surrounding each gradient coil with a shielding coil. A gradient coil and its associated shielding coil make up what is referred to as a “shielded gradient coil set.” The gradient coil may be referred to as the “inner coil,” and the shielding coil may be referred to as the “outer coil.” The function of the shielding coil is to induce electric fields in the region outside of the outer coil. Ideally, the shielding coil is designed to exactly cancel the electric and magnetic field outside of the coil set. Eliminating the field outside of the shielded gradient coil set effectively eliminates the eddy currents that may be induced in the conducting structures of the MRI system, resulting in no error field being produced in the imaging volume.
One problem with this conventional shielding approach is that it is impossible to exactly cancel the field outside of the z-gradient coil set. A continuous surface current distribution would be required on the surface of the shielding coil to exactly cancel the field outside of the gradient coil set. Conventional shielding simulates a continuous surface current distribution by winding several discrete circular loops around a support structure, however, these discrete circular loops cannot exactly simulate a continuous surface distribution, and therefore, never exactly cancel the field outside of the gradient coil set. Another problem with the shielding approach is that it is accomplished at considerable cost in gradient coil build. Furthermore, this practice requires space in the magnet, which further increases the cost of the magnet.
It would therefore be desirable to improve the quality of a magnetic resonance image by producing fields which counteract fields generated by eddy currents in nearby conductors, without employing a shielding coil approach. It would also be desirable to eliminate resultant eddy currents in folded gradient or 3D-gradient designs where it is extremely difficult to design adequate self-shielding, or for designs in which the gradient coil is completely unshielded and effort is made to make the magnet of substantially non-conducting materials.